A painter who hears noise in colors. A musician who sees sounds in shapes. People who taste words. These are just some examples of what it’s like to have synesthesia ― a condition where input received via one sense triggers a response from another sense, causing people to experience reduced boundaries between vision, hearing and other types of perception.

Now, a study published Sept. 15 in the European Journal of Neuroscience suggests that simply having one type of synesthesia ― for example, seeing colors in letters of the alphabet ― is enough to blur the lines between other senses as well.

“It shows that something about their synesthesia is spilling over into another domain,” Dr. Krish Sathian, a neurologist at Emory University, said in a press release.

The findings may help researchers pin down the exact cause of synesthesia, which remains mysterious. Are the brains of people with synesthesia wired differently from everyone else’s? Or are they simply ordinary brains doing what ordinary brains do, only more intensely?

We don’t yet have enough data to say for sure, but the new research suggests it may be the latter. Most people, even those without synesthesia, make some implicit associations between different sensory features. We describe people’s voices as “sharp” or “soft” as if they’re things we can touch. Or we talk of painting a room in “warm” colors.

One famous example of these so-called cross-modal correspondences is the Bouba-Kiki effect, first observed by the psychologist Wolfgang Köhler in 1929. Experiments have shown that no matter what language someone speaks, he or she is more likely to assign the name “bouba” to a round-cornered shape and the name “kiki” to an angular shape than the other way around.

“Some argue that these phenomena form a continuum, with classical synesthesia being an exaggeration of universal cross-modal correspondences,” the researchers note in this week’s paper. “Others contend that the two are quite different.”

Drawn by Andrew Dunn CC BYSA 30
This picture is used as an example to show that people, regardless of their language, tend to attach certain sounds to certain shapes. In one study, American students and Tamil speakers in India both called the angular shape on the left "kiki" and the curvy one on the right "bouba."

The people in the study had the most common form of synesthesia ― a type known as grapheme-color synesthesia, which causes people to perceive colors in letters and numbers.

But during tests, the researchers found that these people are also more likely than their non-synesthete peers to make associations between shapes and meaningless words.

Synesthesia is thought to be rare, but estimates are highly variable and unclear. Anecdotally the condition has been linked to creativity, with some research backing it up. One study in Australia found 24 percent of synesthetes to be involved in an artistic profession. Among the general population, this figure falls to just 2 percent.

It may be possible to learn synesthesia, by consciously trying to associate different sensory modalities, to improve your memory and creativity.

And some researchers argue that true synesthetes, too, have learned the associations during childhood. One study last year found a link between the colors that some people see in letters and the colors of a popular brand of refrigerator magnets.

That might be hard for synesthetes to believe, however, since they often perceive the letters in highly irregular, complex and almost otherworldly hues.